The present invention relates generally to heat exchangers of the type including a fin assembly having passages through which tubing extends, and more particularly to such a heat exchanger wherein gaps between the tubing and the passages are bridged by films which substantially reduce the amount of fungi and bacteria associated with the heat exchanger and to a method of forming a heat exchanger including such films.
One common type of heat exchanger includes a fin assembly having passages through which extend tubes for carrying a heat exchange fluid. This type of heat exchanger is commonly employed, for example, in automotive air conditioners, household air conditioners and heat pumps. A known problem with such heat exchangers is that they have a tendency to accumulate and emit fungi and bacteria. The fungi and bacteria have an adverse effect and, in some instances, a devastating effect on certain persons. The problem has been addressed in the past. The prior art attempts to cure the problem have not been satisfactory to some persons who continue to have debilitating effects when exposed to the bacteria and fungi emitted by such heat exchangers.
The bacteria and fungi are associated with biofilms that accumulate on the heat exchangers; the heat exchangers can be considered as biologic or ecologic devices for this purpose. The biofilms have favorable growth conditions for microbial life, such as bacteria, fungi, yeast and small aquatic animals. The biofilms accumulate layers of living organisms, the food supply for these organisms, as well as organic and/or inorganic salts and other chemicals and/or debris. Biofilms usually occur in the presence of water and growth nutrients for organisms or a single organism. It has been discovered that biofilms exist under aluminum fin collars on copper coils, i.e. tubing, of heat exchangers of the aforementioned type. Surface cleaning liquids are not able to reach such biofilms under the aluminum fin collars.
The water in heat exchangers of domestic or automotive air conditioners or domestic heat exchangers frequently contains bacteria and fungi. Most relatively small heat exchangers employed in domestic and automotive air conditioners and domestic heat exchangers become colonized with mold and/or bacteria after two to three weeks of use. A xe2x80x9csweaty sock smellxe2x80x9d is associated with such mold and/or in connection with such heat exchangers during spring and winter. This is because domestic air conditioners and heat pumps changeover at this time from heating to cooling and vice versa. Also, automotive and domestic air conditioners are usually first put into use in spring.
Most methods previously employed to reduce the odor and reduce the mold and/or bacteria count of heat exchangers of the aforementioned type involve washing the heat exchanger tubing (i.e., coil) with strong acids or alkali, deodorants, perfumes and/or disinfectants. However, such washing techniques are temporary, at best, lasting several days and requiring frequent re-application. In addition to treating the tubing with these materials, it is also necessary to treat surfaces of drain pans and drainage tubes associated with them.
The available cleansers typically have an odor and are not suitable for chemically-sensitive persons and/or have an adverse effect on the heat exchangers. For example, household bleach removes mold odor but corrodes heat exchanger fin assemblies after a few applications. The corroded fin assemblies cause the fin assembly surfaces to be roughened, thereby causing a greater adherence of mold spores to these surfaces. Many persons who suffer from the adverse effects of the bacteria and fungi associated with the mold become symptomatic within a few days after the tubing has been cleaned with bleach. Three to four days, however, is not sufficient time for the mold to regrow. Consequently, the bleach, in all likelihood, is unable to contact all heat exchanger surfaces.
It is, accordingly, an object of the present invention to provide a new and improved heat exchanger of a type including a fin assembly with passages through which tubing carrying a heat exchange fluid extends, and to a method of making same.
Another object of the present invention is to provide a new and improved heat exchanger of the aforementioned type, wherein the amount of fungi and/or bacteria emitted by the heat exchanger is substantially reduced, and to a method of making such a heat exchanger.
A further object of the present invention is to provide a new and improved heat exchanger of the aforementioned type, wherein the amount of fungi and/or bacteria emitted by the heat exchanger is reduced sufficiently to remove debilitating effects some persons have experienced with prior art heat exchangers of this type, and to a method of making such a heat exchanger.
A further object of the invention is to provide a new and improved heat exchanger of the aforementioned type, wherein gaps between the tubing and the passages of the heat exchangers are bridged with a film for preventing substantial build up and/or escape of bacteria and/or fungi, and to a method of making such a heat exchanger.
It is an additional object of the present invention to provide a new and improved heat exchanger including a fin assembly and tubing or coils, wherein the heat exchanger is constructed in such a manner as to substantially prevent the formation of biofilms on surfaces that cannot be reached by cleaning solutions, and to a method of making such a heat exchanger.
The present invention is concerned with heat exchangers of the type including (1) a metal fin assembly having passages therein, and (2) metal tubing which carries heat exchange fluid and is located in the passages, wherein the metal tubing and passages are arranged so there are gaps between exterior surfaces of the tubing and wall surfaces of the passages. In accordance with one aspect of the invention, films bridge the gaps. Preferably, the films (1) are secured to the wall surfaces of the passages and the exterior surfaces of the tubing and (2) are arranged and made of material for substantially preventing transfer of at least one of fungi and bacteria across the gaps.
Preferably, the films include an electrically insulating powder and have a thickness in the range of about 2 to 5 mils. The films are preferably formed by applying the electrically insulative powder to the gaps and the fin assembly and the tubing while (1) the powder is charged to a DC potential and (2) the fin assembly and tubing are at a DC potential sufficiently different from that of the powder to attract the powder to the fin assembly and the tubing, and by fusing the powder applied to the gaps and the fin assembly and the tubing.
While the powder is applied, the tubing and fin assembly are preferably at a temperature which causes the powder to become tacky when it contacts the tubing and fin assembly.
Preferably, exterior surfaces of the tubing outside the fin assembly and the fin assembly exterior surface are coated with the same material that forms the films, a result preferably achieved by simultaneously applying the powder that forms the coating and the film. The films preferably include a material that is at least one of an antifungicidal and antibacterial agent.
A further aspect of the invention relates to a method of forming a film to bridge a gap between a passage in a metal fin assembly of a heat exchanger and metal tubing for carrying heat exchange fluid, wherein the metal tubing extends through the passage. The method comprises applying an electrically insulating, electrically charged powder to the gap while the fin assembly and the tubing are maintained at a DC voltage which attracts the charged powder to the fin assembly and the tubing so that (1) the gap is bridged by the powder and (2) at least portions of the fin assembly and the tubing in the vicinity of the gap are coated with the powder. The powder is fused to the fin assembly and the tubing while the powder is bridging the gap.
Preferably, the tubing and fin assembly are at an elevated temperature sufficient to cause the powder to become tacky during the powder applying step.
The applying and fusing steps are preferably performed a plurality of times so that a film having a first thickness is formed the first time the applying and fusing steps are performed and the film thickness is increased the second time the applying and fusing steps are performed.
A coating in the range of about 2 to 3 mils is preferably applied to the fin assembly and the tubing during each applying step.
The powder is preferably made of a material that melts at a sufficiently low temperature as to have no substantial effect on the mechanical stability of the fin assembly or the tubing and the fusing step heats the powder to its melting temperature causing the powder on at least one of the fin assembly and tubing to melt into the gap.
In one embodiment, the powder comprises primarily urethane which is heated to approximately 375xc2x0 Fahrenheit during the fusing step. The fin assembly and tubing preferably are at about 150xc2x0 Fahrenheit during the applying step, a result achieved by (1) applying a cleaning solvent to the fin assembly and the tubing exterior surfaces and (2) then, immediately prior to applying the powder, drying the fin assembly and the tubing in an oven having a temperature of approximately 350xc2x0 Fahrenheit.